U.S. applications Ser. Nos. 07/884,516 and 07/884,515, which are herein incorporated by reference, describe communications networks for providing multimedia information, such as information programs, to subscribers on demand. To support near term deployment of these communications networks, early stages of the broadband integrated services digital network (BISDN), which employs Asynchronous Transfer Mode (ATM) and Synchronous Optical Network (SONET), have been used as the network backbone. The early stages of BISDN are characterized as supporting high bandwidth and permanent virtual connections which may be eventually replaced by switched virtual connections. It is envisioned that early stages of BISDN will support communication systems transporting multimedia information, which is characterized by bursty communications with large packet sizes and task response requirements.
Under ATM techniques, data is routed from point-to-point between network nodes in self-contained, fixed-length packets of data called cells. The standard cell for broadband transmission systems has been defined by the International Consultative Committee for Telephony and Telecommunications (CCITT) to be 53 octets in length with five octets dedicated to header information. The basic SONET transmission rate, referred to as the STS-1 rate, is defined to be 51.84 Mb/s. Other rates in the hierarchy are defined as STS-N, where N is the whole number multiplier of the STS-1 rate. For example, STS-3 would be equal to 155.52 Mb/s.
FIG. 1 depicts a known communications network 100 which employs BISDN for providing interconnectivity between network nodes. These nodes include a plurality of Information Warehouses (IWHs) 10, a plurality of Central Offices (COs) 40, and a plurality of customer premises equipments (CPEs) 70. Each IWH 10 may be connected to multiple COs 40 via high speed trunks 90 operating at standard broadband rates. These trunks 90 operate at transmission rates which are part of a hierarchy of digital rates, each being a multiple of the basic SONET transmission rate. Multiple COs are also interconnected by high speed trunks 90. Each CO 40 serves multiple CO clusters (CC), each of which is connected to multiple CPEs 70 via transmission links 91 operating bidirectionally at low-speed rates, such as 1.5 Mb/s downstream and a few kilobits/second upstream.
The IWHs archive multimedia and related information and dispense this information to COs as requested. In addition to the functions conventionally provided at COs in broadband networks, COs also manage subscribers' requests for multimedia information, request segments of multimedia from appropriate IWHs, and buffer the segments once received from the IWHs for play-out to requesting subscribers. Each CO also employs ATM switching equipment 42 which routes cells between the CO and destinations in the broadband network. The ATM equipment includes a switch which interconnects each CO cluster to the remainder of the communications network. It is thus possible to send bursts of high-speed digital information between COs and between COs and IWHs.
The CPE is the subscriber's interface to communications network. At the CPE, a subscriber places a request for multimedia information, and once the requested information is received, the CPE presents the information to the subscriber.
FIG. 2 depicts a simplified communications network 300 including a plurality of IWHs 10-1 through 10-N, a CO 40 which includes an ATM switch 42 and M CO clusters 41-1 through 41-M, and CPEs 71-1 through 71-L to illustrate the network operation for providing multimedia information to subscribers. Subscribers place a request for interactive multimedia and also control the play-out of multimedia by interacting with their CPEs, which forward requests to the CO clusters. The CO clusters simultaneously receives multiple requests from CPEs. At each CO, these requests are processed and further requests are sent to appropriate IWHs for information to fulfill subscribers' information needs. To deliver each request to the appropriate IWHs, connections are established between each CO cluster and each IWH archiving requested information via the ATM switch. Therefore, multiple CO clusters could simultaneously send requests in the form of ATM packets to the same IWH. IWHs respond to these requests by sending the requested information to the requesting CO clusters via the ATM switch. Therefore, multiple IWHs may concurrently transmit information to the same CO cluster.
The main problem with this network configuration is that collisions at ATM switch output ports connected to the CO clusters may occur between packets being delivered from multiple IWHs for multiple CPEs located at the same cluster of the CO. For example, assume that IWHs 10-1, 10-2, and 10-N desire to send information to CPEs 71-1, 71-2, and 71-3, respectively, at substantially the same time, which CPEs are connected to CO cluster 41-1 via output port 43-1 of the ATM switch 42. The ATM switch would likely establish connections in rapid succession between each of the IWHs and the CO cluster 41-1 to transmit the requested information. But, generally ATM switches are not designed to buffer large numbers of cells. Furthermore, ATM switches provide no signalling or other intelligence between IWHs and CO clusters for communicating whether the available buffer capacity at the output port of the switch can accommodate the information to be sent from the IWH to the CO cluster. Therefore, the transmission of data from IWHs 10-1, 10-2, and 10-N to the CO cluster 41-1 in rapid succession could result in buffer overflow at output port 43-1 of the ATM switch 42 and information being lost.
In communications networks which deliver interactive multimedia information in real time, uninterrupted information delivery is key, and, therefore, a significant loss of information at an ATM switch output port connecting to a CO cluster cannot be tolerated. Requests received by the IWHs are task oriented and thus require the delivery of the requested information within a specified period of time to ensure uninterrupted service to subscribers. To support the near term delivery of multimedia information consistent with the present and evolving public switched network, it is advantageous to build the communications network utilizing BISDN, the network of the future, as the network backbone, despite the collision problems and shortcomings of the ATM switch. Therefore, an object of our invention is a system and method for interactively delivering multimedia information in real time from IWHs to CO clusters via an ATM switch and for avoiding collisions at the output ports of the ATM switch such that uninterrupted information delivery can be provided. A further objective of our invention is a system and method for scheduling transmission of multimedia information in real-time between nodes of an ATM switch to avoid collisions at the output of the ATM switch.